Energy radiation generator with uni-polar voltage ladder

ABSTRACT

A well-logging tool may include a sonde housing and a radiation generator carried by the sonde housing. The radiation generator may include a generator housing, a target carried by the generator housing, a charged particle source carried by the generator housing to direct charged particles at the target, and at least one voltage source coupled to the charged particle source. The at least one voltage source may include a voltage ladder comprising a plurality of voltage multiplication stages coupled in a uni-polar configuration, and at least one loading coil coupled at at least one intermediate position along the voltage ladder. The well-logging tool may further include at least one radiation detector carried by the sonde housing.

BACKGROUND

Radiation generators, such as neutron and X-ray generators, are used inwell logging tools to take measurements of a geological formationadjacent a wellbore where hydrocarbon resources may be located (e.g.,oil and/or natural gas). Neutron generators may use deuterium-deuterium(d-d), deuterium-tritium (d-t) or tritium-tritium (t-t) reactions tocreate neutrons without the use of radioactive materials.

Radiation generators may include a tube (e.g., a neutron or X-ray tube)and associated electrical components, such as one or more high voltagetransformers with a Cockcroft-Walton ladder to produce a high operatingvoltage. A neutron tube is a sealed envelope made of metal andinsulators including a gas reservoir, an ion source, an acceleratorcolumn and a target. The target may be made of a hydride material. Oncereleased from the reservoir, the gas is ionized in the ion source, andthen accelerated in the accelerator column toward the target. A nuclearfusion reaction occurs between the incoming ions and the hydrogenisotope atoms present in the target, causing neutrons to be directedinto the geological formation. A radiation detector may detect theradiation from the geological formation resulting from the neutronbombardment, which in turn provides information regarding thecomposition of the geological formation.

An X-ray tube has an electron source (often called an electron gun), anacceleration column and a target. The target may be made of a heavymaterial, such as tungsten or gold, for example.

SUMMARY

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofthe claimed subject matter.

A well-logging tool may include a sonde housing and a radiationgenerator carried by the sonde housing. The radiation generator mayinclude a generator housing, a target carried by the generator housing,a charged particle source carried by the generator housing to directcharged particles at the target, and at least one voltage source coupledto the charged particle source. The at least one voltage source mayinclude a voltage ladder including a plurality of voltage multiplicationstages coupled in a uni-polar configuration, and at least one loadingcoil coupled at at least one intermediate position along the voltageladder. The well-logging tool may further include at least one radiationdetector carried by the sonde housing.

A radiation generator may include a generator housing, a target carriedby the generator housing, a charged particle source carried by thegenerator housing to direct charged particles at the target, and atleast one voltage source coupled to the charged particle source. The atleast one voltage source may include a voltage ladder comprising aplurality of voltage multiplication stages coupled in a uni-polarconfiguration, and each multiplication stage may include at least onesemiconductor diode. At least one loading coil may be coupled at atleast one intermediate position along the voltage ladder.

A method for making a radiation generator may include positioning atarget and a charged particle source in a generator housing so that thecharged particle source directs charged particles at the target, andcoupling at least one voltage source to the charged particle source. Theat least one voltage source may include a voltage ladder comprising aplurality of voltage multiplication stages coupled in a uni-polarconfiguration, with each multiplication stage comprising at least onesemiconductor diode, and at least one loading coil coupled at at leastone intermediate position along the voltage ladder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of a radiation generator inaccordance with an example embodiment.

FIG. 2 is a side view of an X-ray tube which may be used in theradiation generator of FIG. 1 in an example embodiment.

FIG. 3 is a schematic block diagram of a well-logging tool which mayinclude a radiation generator as shown in FIG. 1.

FIG. 4 is a schematic diagram of a uni-polar voltage ladderconfiguration which may be used with the radiation generator of FIG. 1.

FIG. 5 is a plot comparing the voltage distribution along variousuni-polar voltage ladder configurations with and without loading coils.

FIG. 6 is a plot of output voltage versus input voltage for theuni-polar voltage ladder configuration of FIG. 1, and for a uni-polarvoltage ladder without added loading coils.

FIG. 7 is a schematic diagram of a bi-polar voltage ladder configurationwhich may be used with the radiation generator of FIG. 1.

FIG. 8 is a schematic circuit diagram of an embodiment of a radiationgenerator and associated control circuitry.

FIG. 9 is a flow diagram illustrating method aspects associated withmaking a radiation generator such as the one shown in FIG. 1.

FIG. 10 is a graph illustrating voltage distribution based uponfrequency variation for a radiation generator in an example testconfiguration.

FIG. 11 is a plot comparing the voltage distribution along a bi-polarvoltage ladder branch with a loading coil and a bi-polar voltage ladderbranch with no loading coil.

DETAILED DESCRIPTION

The present description is made with reference to the accompanyingdrawings, in which example embodiments are shown. However, manydifferent embodiments may be used, and thus the description should notbe construed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete. Like numbers refer to like elements throughout, and primeand multiple prime notation are used to indicate similar elements indifferent embodiments.

Referring initially to FIGS. 1 and 2, a radiation generator 30 is firstdescribed. In the illustrated example, the radiation generator is anX-ray generator which includes an X-ray tube 100 that is grounded at atarget (i.e., anode) end 102, although floating target configurationsmay also be used in some embodiments. The X-ray tube 100 furtherillustratively includes a cathode 103 on the opposite end of the tubefrom the target end 102. The cathode 103 is coupled to a voltagemultiplication ladder 104 (e.g., via a cathode isolation transformer,for example). The X-ray tube 100, voltage multiplication ladder 104, andan isolation transformer 106 are enclosed within one or more insulatingsleeves 108 (e.g., PFA), which in turn is enclosed within a generatorhousing 110. An insulating gas may be inserted in the inner space 117within the generator housing. The voltage multiplication ladder 104further illustratively includes a plurality of loading coils 105 a, 105b, which will be described further below, and an input 116 for receivingan AC voltage. The grounded target configuration shown schematically inFIG. 1 provides a simplification in the mechanical design and assembly,which may also help in maintaining mechanical stability of the target,maintaining thermal management of the target, as well as the radiationexposure of the insulating material 108.

The cathode 103 releases electrons in response to exposure to heat,although in some embodiments “cold” cathodes (e.g., Carbon nanotubes,etc.) may also be used. As will be described further below, voltageladder 104 applies a voltage to the cathode 103, and the introduction ofcurrent heats the cathode 103 and causes it to release electrons. A grid204 moves electrons released from the cathode 103 toward an electronaccelerating section 206. The accelerating section 206 speeds electronstoward a target 208. Upon collision with the target 208, X-rays aregenerated which may be used in various applications, such as downholewell-logging measurements, as will be discussed further below.

A basic uni-polar voltage ladder configuration may be inadequate forachieving very high voltages (e.g., on the order of hundreds of KeV)within the space confines dictated for downhole use. That is, given thespace constraints of the downhole tool pad or sonde housing in which avoltage ladder is deployed, it may be difficult to achieve desiredvoltage levels with the basic uni-polar configuration. Moreparticularly, this is due to voltage efficiency, which may be defined asthe ratio of the output voltage and the input voltage multiplied by thenumber of stages. For example, a 30 or 40 stage basic uni-polar voltageladder will have a voltage efficiency of about 40 to 60%. For an inputvoltage of 15 kV, which is roughly the maximum voltage rating for mostcommercial components (e.g., capacitors and diodes) at reasonable sizes,the output voltage may be plotted against the number of stages.Cascading stages reduces the voltage efficiency. The output voltageconverges to a given value, which is around 250 kV. Adding a relativelylarge number of stages may therefore not provide desired high operatingvoltages. The inability of such configurations to generate high voltagesmay further be attributed to the stray capacitance across the stages.

In order to generate a voltage of 400 kV with a uni-polar ladder (asopposed to a bi-polar design), for example, given the packaging sizeconstraints of downhole equipment, the embodiments set forth hereinprovide for increased voltage efficiency through the use of one or moreloading coils positioned at appropriate intermediate locations orpositions in the ladder. A configuration in which a single boosting, orloading, coil was used in a uni-polar design for ion accelerators andtelevision circuits is set forth in “The Cockcroft-Walton VoltageMultiplying Circuit”, E. Everhart and P. Lorrain, 1953, The Review ofScientific Instruments, Vol. 24, 3, Mar. 1953. This configurationemployed a single coil at the high voltage end of the voltagemultiplier, boosting initial voltage efficiency from 50% to about 80%.With a classic Cockcroft-Walton ladder, the voltage efficiency is givenby:

$\begin{matrix}{{F = \frac{\tanh\left( {2N\sqrt{\frac{C_{s}}{C}}} \right)}{2N\;\sqrt{\frac{C_{s}}{C}}}},} & (1)\end{matrix}$where C is the ladder series capacitor, C_(s) is the stray capacitance,and N is the number of voltage multiplication stages. With theabove-noted single coil positioned at the end of a bi-polar voltageladder, the efficiency becomes:

$\begin{matrix}{F = {\frac{\tanh\left( {N\sqrt{\frac{C_{s}}{C}}} \right)}{N\;\sqrt{\frac{C_{s}}{C}}}.}} & (2)\end{matrix}$By comparing equations with or without a loading coil, there is notablya factor of two difference. That is, the efficiency is the same as auni-polar ladder without a loading coil, yet with two times fewerstages. A voltage distribution for this single, end-connected loadingcoil configuration is represented by plot line 63 in FIG. 5(corresponding to an efficiency of approximately 78%), and a plot line62 represents a uni-polar voltage ladder with no loading coil(corresponding to an efficiency of approximately 50%).

Even so, the voltage efficiency may be further improved by using one ormore loading coils positioned between adjacent voltage multiplicationstages in a voltage multiplication ladder. A first experiment was madewith a first loading coil (0.4 H) at an end of the ladder, and a secondloading coil (0.2 H) in the middle of the ladder, resulting in a voltageefficiency governed by the equation:

$\begin{matrix}{F = \frac{\tanh\left( {\frac{N}{2}\sqrt{\frac{C_{s}}{C}}} \right)}{\frac{N}{2}\;\sqrt{\frac{C_{s}}{C}}}} & (3)\end{matrix}$A voltage distribution for this configuration is represented by the plotline 61 in FIG. 5 (corresponding to an efficiency of approximately 93%).The voltage efficiency of this ladder is also improved over a ladderlacking loading coils, as it is equivalent to a ladder without coils yetwith four times fewer stages.

Further experimentation proved that to have a desired voltageefficiency, the first and second coils 105 a, 105 b may be positionedtwo-fifths and four-fifths stage positions, respectively, down thelength of the ladder 104 (as shown in FIG. 4). More particularly, withthe first coil 105 a and the second coil 105 b, which are substantiallyidentical to one another, respectively positioned at ⅖^(th) and ⅘^(th)along the length of the ladder 104, the voltage efficiency is governedby the equation:

$\begin{matrix}{F = \frac{\tanh\left( {2\frac{N}{5}\sqrt{\frac{C_{s}}{C}}} \right)}{2\frac{N}{5}\;\sqrt{\frac{C_{s}}{C}}}} & (4)\end{matrix}$From the above results it will be understood that a single intermediateloading coil 105 be used in some embodiments, and that the loading coils105 a, 105 b may be located in positions other than the ⅖^(th) and⅘^(th) positions.

A voltage distribution for this configuration is represented by the plotline 60 in FIG. 5 (which corresponds to an efficiency of approximately96%). The efficiency is the same as a ladder without loading coils withfive times less stages. For example, it is estimated that a ladder offorty stages with the two-coil configuration will have the same voltageefficiency as a ladder with no coil but with eight stages. At theoptimum frequency, the input impedance is found to be:

$\begin{matrix}{Z_{in} = {\frac{1}{i\; 2\;\pi\; f\sqrt{{CC}_{S}}{\tanh\left( {2\;\frac{N}{5}\sqrt{\frac{C_{S}}{C}}} \right)}}.}} & (5)\end{matrix}$with f being the optimum frequency. The impedance is then capacitive,with

$\begin{matrix}{C_{eq} = {\sqrt{{CC}_{S}}\mspace{14mu}{{\tanh\left( {2\frac{N}{5}\sqrt{\frac{C_{S}}{C}}} \right)}.}}} & (6)\end{matrix}$The optimum coil values are:

$\begin{matrix}{L_{3} = {{\frac{1}{2\omega^{2}\sqrt{{CC}_{S}}} \times \frac{1}{\tanh\left( {2\frac{N}{5}\sqrt{\frac{C_{S}}{C}}} \right)}\mspace{14mu}{with}\mspace{14mu} L_{1}} = {L_{2}.}}} & (7)\end{matrix}$Hence, the above-described architecture allows for two coils of the samevalue to be used. The optimium frequency therefore will be equal to:

$\begin{matrix}{{f_{opr} = {\frac{1}{2\pi}\sqrt{\frac{1}{L\sqrt{{CC}_{s}}} \times \frac{1}{\tanh\left( {2\frac{N}{5}\sqrt{\frac{C_{s}}{C}}} \right)}}}},{{if}\text{:}}} & (8) \\{{N\sqrt{\frac{C_{s}}{C}}{\operatorname{<<}1}},{f_{opt} \approx {\frac{1}{2\pi}{\sqrt{\frac{5}{2{NLC}_{s}}}.}}}} & (9)\end{matrix}$

At a first order approximation, the optimal frequency does not depend onthe C value. The impact on the voltage distribution of a variation inthe frequency may be seen in FIG. 10, in which a plot 80 represents thevoltage distribution at an optimal frequency of 72.5 kHz with anefficiency of 95%. As represented by plot 81, if the frequency is toolow (e.g., 70 kHz), the voltage efficiency is too high (106%), whichmeans that some voltage multiplication stages will see a voltage higherthan the input voltage. As represented by plot 82, if the frequency istoo high (e.g., 75 kHz), the ladder does not work in its optimal mode,providing a voltage efficiency of 87%, as the voltage on the last stagesis too low. However, it will be understood that an acceptable range offrequency variation (other than just the optimal frequency) may be usedin some embodiments. An example configuration with a uni-polar voltageladder and dual loading coils as described above was constructed andtested. The test configuration included the following:

-   -   30 voltage multiplication stages with 1 nF, X7R capacitors rated        at 16 kV and 16 kV diodes;    -   2×0.2H coils, one being at ⅖^(th) of the ladder and the other        one at ⅘^(th), with an operating frequency of ˜70 kHz;    -   6 PFA insulation sleeves (total thickness of 380 mil) and 3        layers of 20 mil FEP film;    -   a 20 GΩ string of resistors (bleeder) at the end of the ladder        to provide a measurement of the high voltage    -   an X-ray tube; and    -   a 40″ long stainless steel pressure housing with a 3″ OD (2.85″        ID), pressurized with SF6 (around 120 psi).        The input voltage was measured with a 10 GΩ resistor string        connected to the first stage of the ladder. The system was        controlled with Labview. The test configuration was tested up to        400 kV and 40 μA and at elevated temperature. As can be seen by        the results illustrated in FIG. 5, with loading coils in place        in the uni-polar voltage ladder, particularly if placed in the        optimized positions, substantially higher voltages are achieved        for the same number, or fewer, of stages. This configuration may        also be desirable not only in terms of the higher efficiency,        but also since no coils are located on the ends of the voltage        ladder, which may subject them to a greater risk of damage if        arcing occurs.

With reference to FIG. 6, plots 70, 71 of output voltage versus inputvoltage, with and without loading coils added to a uni-polar voltageladder, respectively, further illustrate the benefits of the loadingcoils 105 a, 105 b. This is helpful in terms of feedback and regulation.To stabilize the radiation generator, feedback loops on the inputvoltage, frequency and cathode drive may be used. One example radiationgenerator control configuration is shown in FIG. 8, in which the highvoltage (HV) transformer or driver 306 is coupled to an input of theuni-polar voltage ladder 304, and the output of the uni-polar voltageladder is coupled to a charged particle source 300 (here an X-ray tubeincluding a cathode 320 coupled to an associated cathode driver 321). AnX-ray detector 322 detects X-ray beams from the X-ray tube, andassociated detector acquisition electronics 323 are coupled to the X-raydetector. A microprocessor 324 is coupled to the HV driver 306, an inputvoltage sensor 325, an output voltage sensor 326, the cathode driver321, a cathode current sensor 327, and the detector acquisitionelectronics 323.

More particularly, the microprocessor 324 receives a measured inputvoltage V_(in) to the ladder 304 from the input voltage sensor 325(illustratively represented as a resistor R_(in) and a currentmeasurement I_(in)). Another input to the microprocessor 324 is anoutput voltage V_(out) of the ladder 304 from the output voltage sensor326 (illustratively represented as a resistor R_(out) and a currentmeasurement I_(out)). Other inputs to the microprocessor 324 include antarget control current I from the cathode current sensor 327, as well asestimates of the current I and output voltage V_(out) from the detectoracquisition electronics 323. The microprocessor 324 may accordinglyregulate the HV driver 306 and the cathode driver 321 to maintainconstant values of the output voltage Vout, the current I, and a voltageefficiency value F, where

$F = {\frac{V_{out}}{N \times V_{in}}.}$In an example configuration, it may be desirable for the microprocessor324 to maintain a voltage V_(out)=300 kV, a current I=100 μA, and avalue of F=90%. As noted above, the value of the voltage Vout andcurrent I may be estimated with resistors strings and/or with an X-raydetector measuring both the flux and the energy of the X-ray beam, forexample. The voltage efficiency may be regulated to the desired value byadjusting the frequency of the voltage multiplication ladder 304 HVdriver 306.

The voltage output V_(out) is regulated to the desired value byadjusting the input voltage V_(in). Measuring the input peak-to-peakvoltage V_(in) is performed to adjust the voltage efficiency with thefrequency. Measuring a high voltage AC signal may be difficult as aresult of cross-talk, for example. The input voltage V_(in) mayaccordingly be approximated as the DC voltage on the first capacitor ofthe ladder's DC leg, which is theoretically very close to V_(in). Theoutput voltage V_(out) may be estimated from a string of resistors orfrom a reference detector, as noted above. In parallel, the beam currentis adjusted to the desired value by changing the cathode driver 321. Byusing loading coils 305 a, 305 b in the voltage ladder 304, the voltageefficiency may be boosted from approximately 50% to 95%, which makes a400 kV single-ended ladder and grounded target generator feasible withinthe space constraints of a downhole tool.

The above-described uni-polar voltage ladder configurations may providecertain advantages over bipolar generator configurations in someembodiments. For example, this approach may help to reduce a risk ofarcing, as there is no turn-around at the end of the ladders, andtherefore no standoff to ground at the ends of the generator.Furthermore, high voltage may be confined in the middle of the generatorwith a ground at both ends. This in turn may, reduce the risk of arcingor tracking on the insulating materials (by way of reference, see U.S.Pat. No. 7,564,948, which is also assigned to the present Assignee andis hereby incorporated herein in its entirety by reference). However, insome implementations, a bi-polar configuration with the same potentialdifference between the source and the target may have lower stresses onthe insulation, since the maximum potential difference to ground may beas much as 50% lower, for example.

In addition, a risk of radiation damage may be reduced, as the targetmay be fully shielded, for example, by a tungsten collimator.Furthermore, thermal management of the power on the target may berelatively straightforward, since the target may be attached to a heatsink, for example. In addition, the mechanical design and assembly maybe simplified, which may make it easier to maintain mechanical stabilityof the target, which is a consideration for the accuracy in themeasurement (for example, formation density for X-ray measurements andporosity for neutron measurements).

Additionally, with a grounded target design, the distance between thepoint of emission of X-rays and the detector may be reduced, as the needfor high voltage insulation on the target may be reduced (i.e., on thepositive side of the X-ray tube). In particular, with a uni-polarconfiguration no voltage ladder needs to be positioned between thetarget and the detector, which may help reduce or eliminate high voltageturn-around, provide desired detector to target spacing, and additionalroom for the detector(s). Moreover, in some bipolar designs parasiticphotons may reach the near detectors inside the pad. This may bemitigated by the above-described uni-polar configurations, which offerthe ability to use backscatter-like detectors (e.g., PEx). Furthermore,the beam current (i.e., the flow of electrons in the tube hitting thetarget) may be measured directly.

However, in some embodiments it may be desirable to utilize a bi-polarvoltage ladder configuration with a loading coil(s), as now describedwith reference to FIG. 7. In the illustrated example, the voltage ladderincludes a positive voltage branch 404 p and a negative voltage branch404 n, each of which includes a respective plurality of voltagemultiplication stages 411, which are similar to those described abovewith reference to FIG. 4. Each of the branches 404 p, 404 n has arespective input coupled to a transformer or HV driver 406, and arespective output coupled to a charged particle generator 400 (e.g., anX-ray tube, ion generator, etc.) having a grounded target. In theillustrated example, loading coils 405 p and 405 n are coupled atrespective intermediate positions along each of the positive voltagebranch 404 p and the negative voltage branch 404 n.

More particularly, in the present example the intermediate positions areat one-third N, where N is the total number of the voltagemultiplication stages 411 in both of the positive and negative voltagebranches 404 p, 404 n, which has been determined to provide a desiredvoltage distribution similar to those discussed above for the uni-polarconfigurations. It may be shown that for the bi-polar configuration, thevoltage efficiency is equal to:

$\begin{matrix}{F = {\frac{\tan\left( {2\frac{N}{3}\sqrt{\frac{C_{S}}{C}}} \right)}{2\;\frac{N}{3}\sqrt{\frac{C_{S}}{C}}}.}} & (10)\end{matrix}$By comparing equation (10) with equation (1) (i.e., no coils) or withequation (2) (i.e., a single, end-connected coil), it will beappreciated that the efficiency is improved and is equivalent to aladder with approximately three times fewer stages.

The foregoing will be further understood with reference to FIG. 11, inwhich the voltage distribution of a bi-polar voltage ladder sectionhaving fifteen multiplication stages with a loading coil at the ⅓^(rd)position is shown by a plot line 86 (corresponding to an efficiency ofapproximately 95%), and the voltage distribution of a bi-polar laddersection also having fifteen multiplication but with no loading coil isshown by the plot 87 (corresponding to an efficiency of approximately67%). However, as with the uni-polar configurations described above,different numbers of loadings coils and intermediate positions may beused in different embodiments.

It should be noted that to generate 400 kV with a bi-polar ladder, eachof the positive and negative ladder sections has to generate +200 kV and−200 kV. The respective input voltage with fifteen stages would then be14 kV with one coil at the ⅔^(rd) position (below 15 kV), and 20 kV withno coil (above what is actually achievable with current componenttechnology in a confined space). Moreover, while the bi-polar ladderconfiguration may still utilize a ladder portion adjacent the anode(i.e., target), since the number of stages may be reduced as a result ofthe increased efficiency, this may still provide for increased space forthe detector, as well as shorter distances between the target and thedetector.

Turning now to FIG. 3, an example application of the above-describedradiation generators in a downhole well-logging tool 514 for determiningthe density and other properties of a formation 500 surrounding aborehole 502 is now described. As noted above, the tool 514 ispositioned downhole to determine properties of the formation 500 usinginput radiation that is subsequently detected. In the illustratedembodiment, the tool 514 includes a sonde housing 516 that houses thecomponents that are lowered into the borehole 502. In some embodiments,the sonde housing 516 may be a pad housing. Furthermore, a mandrel-typepressure housing may also be used for implementations such as wireline,slickline, CTD, TLC, etc. In another example configuration, the sondehousing 516 may be a collar to be carried by a Logging While Drilling(LWD) tool assembly or string, and the radiation generator may becarried or located in the chassis inside the collar, for example.

A radiation generator 512, such as those described above (e.g., X-ray,neutron, etc.) introduces radiation into the formation 500. Theradiation is to some extent scattered from different depths in theformation 500, and the resultant radiation signal is detected by a shortspaced detector 510 and a long spaced detector 506, for example,although other detector configurations may be used in variousembodiments.

During the drilling process, the borehole may be filled with drillingmud. The liquid portion of the drilling mud flows into the formation500, leaving behind a deposited layer of solid mud materials on theinterior wall of the borehole in the form of mudcake 518. For reasonsdescribed below, it may be desirable to position the radiation generator512 and detectors 506, 510 as close to the borehole wall as possible fortaking measurements. Irregularities in the wall of the borehole maycause measurement degradation as the sonde housing 516 becomes longer,so it may be desirable to keep the entire tool 514 as short in length aspossible. The sonde housing 516 is lowered into position and thensecured against the borehole wall through the use of an arm 508 and asecuring skid 524, for example. The tool 514, in one embodiment, islowered into the borehole 502 via a wireline 520. Data is passed back toan analysis unit 522 for determination of formation properties. The tool514 may be used downhole for wireline, logging-while-drilling (LWD),measurement-while-drilling (MWD), production logging, and permanentformation monitoring applications, as noted above, for example.

A method of making radiation generators, such as those set forth above,is now described with reference to the flow diagram 700 of FIG. 9.Beginning at Block 701, a generator tube (e.g., an X-ray or neutrontube) is positioned in a generator housing 110 including a target andcharged particle source, as described above, at Block 702. Additionally,at least one voltage source is coupled to the charged particle source,at Block 703. As noted above, the voltage source includes a voltageladder 104 including a plurality of voltage multiplication stagescoupled in a uni-polar (or bi-polar) configuration, and one or moreloading coils 105 coupled at at least one intermediate position alongthe voltage ladder. The method concludes at Block 704.

As noted above, the above-described radiation generators may be usedwith both grounded target and floating target configurations. For mostuni-polar neutron generator applications, the target is at a negativehigh voltage, while the ion source is virtually at ground. In X-raytubes, it may be helpful to have the target at ground potential and theelectron source at a high negative potential. In a bi-polar design, boththe target and ion source may be floating, for example. In a minitronconfiguration, either the target or the ion source may be grounded, withthe diodes in the voltage multiplication ladder oriented (or inverted)appropriately.

Many modifications and other embodiments will come to the mind of oneskilled in the art having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it isunderstood that various modifications and embodiments are intended to beincluded within the scope of the appended claims.

That which is claimed is:
 1. A well-logging tool comprising: a sonde housing; a radiation generator carried by said sonde housing and comprising a generator housing, a target carried by said generator housing, a charged particle source carried by said generator housing to direct charged particles at said target, and at least one voltage source coupled to said charged particle source, said at least one voltage source comprising a voltage ladder comprising a plurality of voltage multiplication stages coupled in a unipolar configuration, and at least one loading coil coupled at at least one intermediate position along said voltage ladder; wherein said at least one voltage source has an operating frequency and a voltage efficiency, wherein said operating frequency is configured to regulate said voltage efficiency to a value between eighty-seven percent and one-hundred-and-six percent; and at least one radiation detector carried by said sonde housing.
 2. The well-logging tool of claim 1 wherein said at least one loading coil comprises a plurality thereof spaced apart at respective intermediate positions along said voltage ladder.
 3. The well-logging tool of claim 2 wherein one of said loading coils is coupled at an intermediate position defined by two-fifths N, where N is a number of said voltage multiplication stages.
 4. The well-logging tool of claim 2 wherein one of said loading coils is coupled at an intermediate position defined by four-fifths N, where N is a number of said voltage multiplication stages.
 5. The well-logging tool of claim 1 wherein each of said voltage multiplication stages comprises at least one semiconductor diode.
 6. The well-logging tool of claim 1 wherein said radiation generator further comprises: a voltage driver coupled to said voltage ladder; at least one voltage sensor coupled to said voltage ladder; and a processor to control said voltage driver based upon said at least one voltage sensor.
 7. The well-logging tool of claim 6 wherein said at least one voltage sensor comprises an input voltage sensor coupled to a first one of said voltage multiplication stages.
 8. The well-logging tool of claim 1 wherein said charged particle source comprises an electron stream generator.
 9. The well-logging tool of claim 1 wherein said charged particle source comprises an ion stream generator.
 10. The well-logging tool of claim 1 wherein said voltage ladder comprises a Cockcroft-Walton voltage ladder.
 11. The well-logging tool of claim 1 wherein said a sonde housing comprises a pad housing.
 12. The well-logging tool of claim 1 wherein said a sonde housing comprises a mandrel housing.
 13. The well-logging tool of claim 1 wherein said sonde housing comprises a collar to be carried by a Logging While Drilling (LWD) tool assembly; and wherein said collar comprises a chassis, and said radiation generator is carried in said chassis.
 14. A radiation generator comprising: a generator housing; a target carried by said generator housing; a charged particle source carried by said generator housing to direct charged particles at said target; and at least one voltage source coupled to said charged particle source, said at least one voltage source comprising a voltage ladder comprising a plurality of voltage multiplication stages coupled in a unipolar configuration, each multiplication stage comprising at least one semiconductor diode, and at least one loading coil coupled at at least one intermediate position along said voltage ladder, wherein said voltage ladder has an operating frequency and a voltage efficiency, wherein said operating frequency is configured to regulate said voltage efficiency to a value between eighty-seven percent and one-hundred-and-six percent.
 15. The radiation generator of claim 14 wherein said at least one loading coil comprises a plurality thereof spaced apart at respective intermediate positions along said voltage ladder.
 16. The radiation generator of claim 15 wherein one of said loading coils is coupled at an intermediate position defined by two-fifths N, where N is a number of said voltage multiplication stages.
 17. The radiation generator of claim 15 wherein one of said loading coils is coupled at an intermediate position defined by four-fifths N, where N is a number of said voltage multiplication stages.
 18. The radiation generator of claim 14 further comprising: a voltage driver coupled to said voltage ladder; at least one voltage sensor coupled to said voltage ladder; and a processor to control said voltage driver based upon said at least one voltage sensor.
 19. The radiation generator of claim 18 wherein said charged particle source comprises an electron stream generator.
 20. A method for making a radiation generator comprising: positioning a target and a charged particle source in a generator housing so that the charged particle source directs charged particles at the target; and coupling at least one voltage source to the charged particle source, the at least one voltage source comprising a voltage ladder comprising a plurality of voltage multiplication stages coupled in a unipolar configuration, each multiplication stage comprising at least one semiconductor diode, and at least one loading coil coupled at least one intermediate position along the voltage ladder, wherein the voltage ladder comprises an operating frequency and a voltage efficiency, wherein the operating frequency is configured to regulate the voltage efficiency to a value between eighty-seven percent and one-hundred-and-six percent.
 21. The method of claim 20 wherein the at least one loading coil comprises a plurality thereof spaced apart at respective intermediate positions along the voltage ladder.
 22. The method of claim 21 wherein one of the loading coils is coupled at an intermediate position defined by two-fifths N, where N is a number of the voltage multiplication stages.
 23. The method of claim 21 wherein one of the loading coils is coupled at an intermediate position defined by four-fifths N, where N is a number of the voltage multiplication stages.
 24. The well-logging tool of claim 1, wherein said operating frequency is not equal to a resonant frequency of said at least one voltage source. 